Color-stable polycarbonate composition and articles molded therefrom

ABSTRACT

A thermoplastic polycarbonate molding composition free from organo phosphorus compounds and having an improved resistance to discoloration caused by exposure to High Intensity Discharge (HID) light and heat is disclosed. Accordingly, the molding composition contains (co)polycarbonate resin, a dimeric benzotriazole and an ester of a 3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid. In a preferred embodiment, a sheet made of (co)polycarbonate resin is rendered improved resistance to discoloration by having applied to at least one of its surfaces an adherent protective film prepared from the polycarbonate composition of the invention.

FIELD OF THE INVENTION

The invention relates to a polycarbonate composition and to articlesmolded therefrom; more particularly the invention relates tocompositions and articles which have an improved resistance todiscoloration caused by exposure to High Intensity Discharge (HID) lightand heat.

SUMMARY OF THE INVENTION

A polycarbonate composition having an improved resistance todiscoloration caused by exposure to High Intensity Discharge (HID) lightand heat is disclosed. Accordingly, the composition which is free fromhydroxyphenyl organophosphorus compounds, contains polycarbonate resin,a dimeric benzotriazole and an ester of a3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid. In a preferredembodiment, a (co)polycarbonate sheet is rendered improved resistance todiscoloration by having applied to at least one of its surfaces anadherent film comprising the polycarbonate composition of the invention.

BACKGROUND OF THE INVENTION

Polycarbonate resins are characterized by their transparency, mechanicalstrength and dimensional stability. These properties make polycarbonatean ideal choice for the preparation of articles, especially laminates orsheets, useful in application in hostile environments. One of thedrawbacks of the resin used in this application is its tendency todiscolor upon prolonged exposure to the heat and/or UV. Among therelevant applications, mention may be made of lighting lenses andassociated parts in conjunction with metal halide, mercury vapor, highpressure sodium and other High Intensity Discharge (HID) lamps whichgenerate UV radiation and significant heat. The tendency ofpolycarbonate to discolor, limits its applicability and methods toovercome the deficiency have long been sought.

It is therefore an object of the present invention to providepolycarbonate compositions which are suitable for the preparation ofarticles, most especially sheets and laminates, having improvedresistance to discoloration caused by exposure to heat and/or UV.

The art has long recognized the efficacy of hindered phenols asantioxidants in the context of polymeric resins. U.S. Pat. No. 4,563,516disclosed sterically hindered phenols as stabilizers of a carbonatepolymer, and a hindered phenol phosphite was disclosed in U.S. Pat. No.4,276,233 as a thermal stabilizer of polycarbonate resins. U.S. Pat. No.4,812,498 disclosed a polycarbonate resin composition having improvedresistance to deterioration when exposed to light and containing thebisbenzotriazole stabilizer of the present invention. Importantly, thepossible use of the bis-benzotriazole in combination with phenolicantioxidants is disclosed in the '498 document.

The art is noted to include Canadian Patent 1,208,873 which disclosed apolycarbonate-based panel made resistant to UV radiation. Accordingly, apanel is structured to include a core layer of polycarbonate to whichthere is adhesively bonded an intermediate UV absorption layer and acover layer. The purpose of the cover layer is to prevent vaporizationof the UV absorber from the intermediate layer. The intermediate,UV-absorption layer may be prepared from polycarbonate and containsderivatives of benzotriazole as UV absorbers. Also noted is GermanPatent Application 1,670,951 which disclosed polycarbonate moldedarticles, including ribbons which are rendered resistant to UV radiationby incorporating the bis-benzotriazole compound of the present inventiontherewith. A method for coating a polycarbonate sheet with a protectivelayer was disclosed in UK Patent Application 2,028,228. A layerpreferably of polymethacrylate and advantageously containing a UVabsorber is said to be applied to the sheet by co-extrusion. U.S. Pat.No. 3,892,889 discloses UV stabilized polycarbonate moldings, thesurfaces of which have been treated with a solution containing abenzotriazole. German DE-OS 3,617,978 discloses co-extruded sheets basedon a polycarbonate resin which sheets are covered by a UV absorbinglayer made from a branched polycarbonate resin containing thebis-benzotriazole of the present invention. Also relevant is EuropeanPatent Application 110,221 which disclosed a panel consisting of a corelayer of polycarbonate containing not more than 0.5 wt. percent of a UVabsorber and having on at least one side a coating layer which has beencoextruded with the core and which contains at least 3 percent of a UVabsorber.

U.S. Pat. No. 4,948,666 is noted to disclose a polycarbonate compositioncontaining the bis-benzotriazole of the present invention useful in thepreparation of a stain-protective layer for polycarbonate sheets. Alsorelevant in the present context is U.S. patent application Ser. No.07/732,262 filed Jul. 18, 1991 which disclosed relevant technology.

The present invention resides in the finding of a particularly efficientcombination of stabilizers.

DETAILED DESCRIPTION OF THE INVENTION

The (co)polycarbonate resins useful in the practice of the invention arehomopolycarbonate, copolycarbonate and terpolycarbonate resins ormixtures thereof. Preferably, the (co)polycarbonate resins havemolecular weights of 18,000-200,000 (weight average molecular weight),more preferably 20,000-80,000, and may alternatively be characterized bytheir melt flow of 1-65 gm/10 min. at 300° C. per ASTM D-1238. These(co)polycarbonates may be prepared, for example, by the known diphasicinterface process from phosgene and dihydroxy compounds bypolycondensation (see German DOS 2,063,050; 2,063,052; 1,570,703;2,211,956; 2,211,957 and 2,248,817 and French Patent 1,561,518 and themonograph, H. Schnell, Chemistry and Physics of Polycarbonates,Interscience Publishers, New York 1964, all incorporated herein byreference).

In the present context, dihydroxy compounds suitable for the preparationof the (co)polycarbonates of the invention conform to the structuralformulae (1) or (2)

wherein

A denotes an alkylene group with 1 to 8 carbon atoms, an alkylidenegroup with 2 to 8 carbon atoms, a cycloalkylene group with 5 to 15carbon atoms, a cycloalkylidene group with 5 to 15 carbon atoms, acarbonyl group, an oxygen atom, a sulfur atom, an —SO— or —SO₂-radical;or a radical of the general formula

g denotes the number 0 or 1;

e denotes the number 0 or 1;

Z denotes F, Cl, Br or a C₁-C₂ alkyl and if several Z radicals aresubstituents in one aryl radical, they may be identical or different;

d denotes 0 or an integer of from 1 to 4; and

f denotes 0 or an integer of from 1 to 3.

Among the useful dihydroxy compounds in the practice of the inventionare hydroquinone, resorcinol, bis-(hydroxyphenyl)-alkanes,bis(hydroxyphenyl)-cycloalkanes, bis-(hydroxyphenyl)-ethers,bis-(hydroxyphenyl)-ketones, bis-(hydroxyphenyl)-sulfoxides,bis-(hydroxyphenyl)-sulfones andα,α-bis-(hydroxyphenyl)-diisopropylbenzenes. These and further suitablearomatic dihydroxy compounds are described, for example, in U.S. Pat.Nos. 3,028,365; 2,999,835; 3,148,172; 3,271,368; 2,991,273; 3,271,367;3,280,078; 3,014,891 and 2,999,846 (all incorporated herein byreference), in German Offenlegungsschriften (German PublishedSpecifications) 1,570,703; 2,063,050; 2,063,052; 2,211,956 and2,211,957, in French Patent Specification 1,561,418 and in themonograph, H. Schnell, Chemistry and Physics of Polycarbonates,Interscience Publishers, New York, 1964. Further examples of suitabledihydroxy compounds are 2,2-bis-(4 -hydroxyphenyl)-propane(bisphenol A),2,4-bis-(4-hydroxyphenyl)-2-methyl-butane,1,1-bis-(4-hydroxyphenyl)-cyclohexane, 2,2,4-trimethylcyclohexyl1,1-diphenol, α,α-bis-(4-hydroxyphenyl)-p-diisopropylbenzene,2,2-bis-(3-chloro-4-hydroxyphenyl)-propane, hydroxybenzo-phenone and4,4′-sulfonyl diphenol; the most preferred one is 2,2-bis-(4-hydroxyphenyl)-propane(bisphenol A).

The (co)polycarbonates of the invention may entail in their structure,units derived from one or more of the suitable dihydroxy compounds.

The preparation of (co)polycarbonate resins may be carried out inaccordance with any of the processes known in the art, for example, bythe interfacial polycondensation process, polycondensation in ahomogeneous phase or by transesterification.

The suitable processes and the associated reactants, catalysts, solventsand conditions are known in the art and have been described, inter alia,in German Patent Nos. 1,046,311 and 962,274 and in U.S. Pat. Nos.3,248,414; 3,153,008; 3,215,668; 3,187,065; 3,028,365; 2,999,846;2,999,835; 2,964,974; 2,970,137; 3,912,638 and 1,991,273.

Monofunctional reactants, such as monophenols, may be used in thepreparation of the (co)polycarbonate resins of the invention in order tolimit their molecular weights. Also optionally used in the preparationof the (co)polycarbonate resin which is suitable for the preparation oflaminates and/or the protective film of the invention are branchingagents of the type described below. Branching may be obtained by theincorporation of small amounts, preferably of between about 0.05 and 2.0mol % (relative to diphenols employed), of trifunctional or more thantrifunctional compounds, especially compounds having three or morearomatic hydroxyl groups. Polycarbonates of this type are described, forexample, in German Offenlegungsschriften (German PublishedSpecifications) 1,570,533; 1,595,762; 2,116,974 and 2,113,347; BritishSpecification 1,079,821 and U.S. Pat. No. 3,544,514 (incorporated hereinby reference).

Some examples of compounds with three or more than three phenyl hydroxylgroups which can be used are phloroglucinol, 4,6-dimethyl-2,4,6-tri-(4-hydroxyphenyl)-heptane,2,4,6-trimethyl-2,4,6-tri-(4 -hydroxyphenyl)-heptane,1,4,5-tri-(4-hydroxyphenyl)-benzene, 1,1,-tri-(4 -hydroxyphenyl)-ethane,tri-(4-hydroxyphenyl)-methane, 2,2-bis[4,4-bis-(4-hydroxyphenyl)-cyclohexyl]-propane,2,4-bis-(4-hydroxyphenylisopropyl)phenol,2,6-bis-(2-hydroxy-5′-methylbenzyl)-4-methylphenol,2-(4-hydroxyphenyl)-2-(2,4-dihydroxyphenyl)-propane,hexa-(4-(4-hydroxyphenylisopropyl)phenyl)-orthoterephthalic acid ester,tetra-(4-hydroxyphenyl)-methanetetra-(4-(4-hydroxyphenylisopropyl)phenoxy)-methane and1,4-bis-((4′,4″-dihydroxytriphenyl)-methyl)-benzene. Some of the othertrifunctional compounds are 2,4-dihydroxybenzoic acid, trimesic acidcyanuric chloride and 3,3-bis-(4-hydroxyphenyl-2-oxo-2,3-dihydroindole.

Among the resins suitable in the practice of the invention are includedphenolphthalein-based polycarbonates, copolycarbonates andterpolycarbonates such as are described in U.S. Pat. Nos 3,036,036 and4,210,741, both incorporated by reference herein.

The bis-benzotriazole of the present invention conforms to formula (3).

where Bridge is either

in which case it is preferred that the OH is in an ortho-position to thebridging group or bridge is

where p is an integer of 0 to 3, q is an integer of 1 to 10 and Ydenotes

in which case it is preferred that the OH group be in a para-position tothe bridging group. In the above formula R¹, R¹¹, R² and R²² independentof each other are a hydrogen or a halogen atom, a C₁-C₁₂ alkoxy, C₇-C₁₈arylalkoxy or a C₁-C₁₀ alkyl, cycloalkyl, aralkyl or an aryl radical andR³ and R⁴ independent of each other are a hydrogen atom, a C₁-C₁₀ alkyl,cycloalkyl, arylalkyl or an aryl radical, n is an integer of 0 to 4 andm is an integer of 1 to 3.

In a more preferred embodiment where —Bridge— is

R³ and R⁴ are hydrogen atoms, n is O, m is 1 and R² is a tertiary octylradical. Another preferred embodiment is represented by abisbenzotriazole where—Bridge—denotes

and is para-positioned to the OH groups and R² is a tertiary butyl,orthopositioned to the hydroxyl groups.

The most preferred dimeric benzotriazole suitable in the practice of theinvention conforms structurally to

The stabilizer in the context of the invention is an ester of a3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid conforming structurally to

wherein R is a linear or branched C₁₋₂₄-alkyl radical, preferablyC₁₄₋₂₂-alkyl radical. In a most preferred embodiment R is —C₁₈H₃₇.

The composition of the invention is free from organophosphorouscompounds, and contains about 1.0 to 30, preferably about 1.0 to 15.0%of the bisbenzotriazole and about 0.1 to 5, preferably about 0.1 to 3.0%of the stabilizer, the percentages being relative to the weight of thecomposition.

The preparation of the inventive composition follows conventionalprocedures. The composition of the invention is suitable for thepreparation of a variety of articles by thermoplastic molding, includingextrusion and injection molding. Included among the applications forwhich the inventive composition is useful are sheets and laminates.

In a yet additional embodiment of the invention, a thermally stablelaminate is prepared comprising a (co)polycarbonate sheet having athickness of about 0.8 to 13.0 millimeter, and an adherent, protectivefilm having a thickness of about 0.05 to 1.2 millimeter adherent to atleast one surface of said sheet. The protective film is made of theinventive composition. In a preferred embodiment, the laminate isprepared by coextrusion of the sheet and protective film.Advantageously, the inventive composition of the protective filmcontains linear polycarbonate resin.

The laminate may be prepared conventionally, preferably by coextrusionin a known manner. It may be clear or pigmented, pigmentation attainedby known means. Extrusion of polycarbonates as a method for formingsheets as well as coextrusion to form laminates are known and havepreviously been disclosed in the art.

In a preferred embodiment, the sheet and adherent layer are coextrudedby known teachings and their surfaces are brought into contact at anelevated temperature resulting from the extrusion optionally incombination with the application of pressure, to effect adhesionresulting in the formation of a laminate.

The invention is further illustrated, but is not intended to be limitedby the following examples in which all parts and percentages are byweight unless otherwise specified.

EXAMPLES

The exposure to HID light which normally causes discoloration inarticles molded of (co)polycarbonate may be carried out in an exposurechamber known as “Whirly-GIG-III” available from Electrical ConceptsNorh, 12800 Taylor Road Charlevoix, Mi. 49720. The chamber includes arotating specimen rack, thermostatic heat control and a 400 watt metalhalide lamp. Test plaques are positioned on the rotating platform sothat they are exposed directly to the arc tube of the lamp. The platformrotates at one rpm around the stationary lamp. The plaques arepositioned about 6 inches from the bulb. An external source of heat isprovided for permitting the temperature of the test specimens to be inthe range of 90°-130° C.

Experimental:

Compositions in accordance with the invention were prepared and theirproperties determined as noted below. In preparing the compositions ofthe invention and the control compositions, the resins and the additiveswhich were used were as follows:

(i) Polycarbonate Resin: a bisphenol-A based homopolycarbonate having amelt flow rate of about 6.0 g/10 min. determined in accordance with ASTMD-1238; available as Makrolon 3108 resin, from Miles Inc.

(ii) Dimeric Benzotriazole conforming to

where R² denotes t-octyl group and m is 1.

(iii) as thermal stabilizer one of the following compounds was used:

(a) octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate (herein HPA)conforming to

(b) 3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid triester with1,3,5,-tris(2-hydroxyethyl)-S-triazine-2,4,6,(1H,3H,5H)-trione (hereinHPB) conforming to

(c) thiodiethylene bis-(3,5-di-tert-butyl-4-hydroxy)-hydrocinnamate(herein HPC) conforming to

Each of protective films A1 to A4 in the tables below contained 10% ofthe dimeric benzotriazole and A2, A3 and A4 also contained 1% of thenoted stabilizer, as indicated in the table. The noted percents arerelative to the weight of the composition. Laminates of protective filmsand protected sheets were prepared by conventional methods which arewell known in the art. The protective films (5 mils in thickness) madeof the noted compositions were applied to the protected sheets, that ispolycarbonate substrates (100 mils in thickness) to produce laminates.The laminates were exposed to HID at 105° C. for 2000 hours and theyellowness indices determined periodically as noted in Table 1 below.

TABLE 1 yellowness index of the laminate (protected sheet) afterexposure to HID for the indicated number of hours laminate 0 1000 15002000 ΔY.I₂₀₀₀ A1 3.1 7.0 24.0 36.8 33.7 A2-HPA 4.4 11.4 17.0 24.8 20.4A3-HPB 4.1 9.4 25.5 38.6 34.5 A4-HPC 3.6 9.1 24.4 35.8 32.2

Clearly the effectiveness of protective films made of the composition ofthe invention is demonstrated by the results shown above. The laminatedesignated A2-HPA shows a considerably greater resistance to yellowingthan do any of the other laminates.

Table 2 below reports the results of evaluation of the composition ofthe invention as protective film of substrate made of a copolycarbonateresin. The copolycarbonate used in this series of experiments was onederived from 65 mol % bisphenol A and 35 mol % of trimethylcyclohexanebisphenol. The resin has a melt flow rate of about 4.0 g/10 min. at 320°C. as determined in accordance with ASTM D 1238. The resin is availablecommercially from Miles Inc. as Apec HT DP9-9350 resin.

The compositional makeup of the protective film was as noted above forthe series reported in Table 1. The laminates (protected sheets) wereexposed to HID at 120° C. for 2000 hours and the yellowness indicesdetermined periodically as noted below.

TABLE 2 yellowness index of the laminates (protected sheets) afterexposure to HID for the indicated number of hours laminate 0 1000 15002000 ΔY.I₂₀₀₀ C1 8.6 19.6 42.3 74.4 65.8 C2-HPA 8.2 14.0 23.4 39.6 31.4C3-HPB 8.1 21.3 44.9 75.2 67.1 C4-HPC 7.8 21.0 43.7 74.1 66.3

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

What is claimed is:
 1. A thermoplastic molding composition free fromorganic phosphorous compounds, comprising polycarbonate(co)polycarbonate resin the structure of which entail units derived fromone or more bis(hydroxyphenyl)-cycloalkane and (i) about 1.0 to 30percent of a bis-benzotriazole conforming structurally to

wherein R¹ and R² independent of each other are a hydrogen or a halogenatom, a C₁-C₁₂ alkoxy, C₇-C₁₈ arylalkoxy or a C₁-C₁₀ alkyl, cycloalkyl,arylalkyl or an aryl radical and (Bridge) is either

where p is 0 to 3, q is 1 to 10, Y denotes any of —CH₂—CH₂—,

R³ and R⁴ independent of each other are a hydrogen atom, a C₁-C₁₀ alkyl,cycloalkyl, arylalkyl or an aryl radical, and (ii) about 0.1 to 5.0percent of a stabilizer conforming to

wherein R is a linear or branched C₁₋₂₄-alkyl radical, said percent,both occurrences, being in relation to the weight of said composition.2. The thermoplastic molding composition of claim 1 wherein said (i) ispresent in an amount of about 1.0 to 15 percent, and said (ii) ispresent in an amount of about 0.1 to 3.0 percent, said percent, bothoccurrences, being in relation to the weight of said composition.
 3. Thecomposition of claim 1 wherein said benzotriazole conforms to


4. The composition of claim 3 wherein said stabilizer conforms to


5. A thermally stable laminate comprising (a) a (co)polycarbonate sheethaving a thickness of about 0.8 to 13.0 millimeter, and (b) a protectivefilm having a thickness of about 0.05 to 1.2 millimeter adherent to atleast one surface of said sheet, containing the composition of claim 1.6. The laminate of claim 5 prepared by co-extrusion.
 7. A thermallystable laminate comprising (a) a (co)polycarbonate sheet having athickness of about 0.8 to 13.0 millimeter, and (b) a protective filmhaving a thickness of about 0.05 to 1.2 millimeter adherent to at leastone surface of said sheet, containing the composition of claim
 3. 8. Thelaminate of claim 7 wherein the stabilizer conforms to


9. The laminate of claim 7 made by co-extrusion.
 10. The laminate ofclaim 8 made by co-extrusion.
 11. The thermoplastic molding compositionof claim 1 wherein said units are derived from at least one memberselected from the group consisting of 1,1-bis-( 4-hydroxyphenyl)-cyclohexane and 2,2,4 -trimethylcyclohexyl 1,1-diphenol.